# std::variant<Types...>::emplace

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< cpp‎ | utility‎ | variant

template< class T, class... Args > T& emplace( Args&&... args ); | (1) | (since C++17) (constexpr since C++20)
template< class T, class U, class... Args > T& emplace( std::initializer_list<U> il, Args&&... args ); | (2) | (since C++17) (constexpr since C++20)
template< std::size_t I, class... Args > std::variant_alternative_t<I, variant>& emplace( Args&&... args ); | (3) | (since C++17) (constexpr since C++20)
template< std::size_t I, class U, class... Args > std::variant_alternative_t<I, variant>& emplace( std::initializer_list<U> il, Args&&... args ); | (4) | (since C++17) (constexpr since C++20)

Creates a new value in-place, in an existing `variant` object

1) Equivalent to emplace<I>(std::forward<Args>(args)...), where `I` is the zero-based index of `T` in `Types...`.

- This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true, and `T` occurs exactly once in `Types...`.

2) Equivalent to emplace<I>(il, std::forward<Args>(args)...), where `I` is the zero-based index of `T` in `Types...`.

- This overload participates in overload resolution only if std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true, and `T` occurs exactly once in `Types...`.

3) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type `T_I` with the arguments std::forward<Args>(args).... If an exception is thrown, *this may become `valueless_by_exception`.

- This overload participates in overload resolution only if std::is_constructible_v<T_I, Args...> is true.
- It is a compile-time error if `I` is not less than sizeof...(Types).

4) First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type `T_I` with the arguments il, std::forward<Args>(args).... If an exception is thrown, *this may become `valueless_by_exception`.

- This overload participates in overload resolution only if std::is_constructible_v<T_I, std::initializer_list<U>&, Args...> is true.
- It is a compile-time error if `I` is not less than sizeof...(Types).

### Parameters

args | - | constructor arguments to use when constructing the new value
il | - | initializer_list argument to use when constructing the new value

### Return value

A reference to the new contained value.

### Exceptions

1-4) Any exception thrown during the initialization of the contained value.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_variant` | `202106L` | (C++20) (DR) | Fully constexpr `std::variant` (1-4)

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <variant>

    int main()
    {
        std::variant<std::string> v1;
        v1.emplace<0>("abc"); // OK
        std::cout << std::get<0>(v1) << '\n';
        v1.emplace<std::string>("def"); // OK
        std::cout << std::get<0>(v1) << '\n';

        std::variant<std::string, std::string> v2;
        v2.emplace<1>("ghi"); // OK
        std::cout << std::get<1>(v2) << '\n';
        // v2.emplace<std::string>("abc"); -> Error
    }

Output:

    abc
    def
    ghi

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2231R1 | C++20 | `emplace` was not constexpr while the required operations can be constexpr in C++20 | made constexpr

### See also

operator= | assigns a `variant` (public member function)

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